Fix search response aggregation unmarshalling

[rubenpoppe]

Fixes #660

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
66477eb

Please, read and sign the above mentioned agreement if you want to contribute to this project

[Anaethelion]

Closing in favor of #662